Two engines producing a same thrust as a single equivalent engine are generally heavier, have more drag and nearly double maintenance. As engine efficiency and performance technology plateaus in a presence of increasing range, improved operating economics, and more stringent noise requirements, greater emphasis may be placed on designing single engine aircraft. This has become more compelling as engines have also advanced to become more reliable and a corresponding likelihood of non-optimal aircraft operation in-service due to single engine anomaly decreases.
While these principles are changing for designs of many types of aircraft, these principles are especially changing for smaller aircraft such as Unmanned Arial Vehicles (UAV's), where weight and balance sensitivities can be much greater than for larger aircraft due to relative differences in mass ratios between vehicle, fuel, propulsion system and payload. Accordingly, current art for small UAV's reflect significant usage of designs with single engine propulsion systems utilizing un-shrouded propellers to maximize efficiency and flight time. However, propellers are noisy, and are not easily modified to meet more stringent noise requirements. Increasing a propeller diameter to decrease noise creates complex integration challenges that can require radical changes in vehicle design. Furthermore, propellers may have difficulty in achieving sufficiently low noise levels due to difficulties imposed on a vehicle by having to increase propeller diameter to reduce noise, and where on the vehicle a large diameter propeller has to be installed. For example, an increase propeller diameter can create adverse installation effects which counter-act noise reduction from the increased diameter.